1. Field of the Invention
This invention relates generally to an automatic transportation system for transporting materials in factories, warehouses or the like, and more particularly to an apparatus for positioning a carriage in such an automatic transportation system.
2. Prior Art
There has been proposed an automatic transportation system for transporting materials for an assembly line in a factory which comprises a carriage or a pallet propelled by a linear induction motor (hereinafter referred to as "LIM"). Such as automatic transportation system includes a free-roller conveyor laid on a floor, a plurality of stators of the LIM mounted on the floor along the free-roller conveyor, and a carriage (or a pallet) placed on the free-roller conveyor for slidable movement and provided at its bottom surface with a secondary conductor which serves as a reaction plate of the LIM. The stators are suitably energized to move the carriage to a predetermined position or station to thereby transport goods placed on the carriage.
With such an automatic transportation system as described above, to stop the moving carriage, the stators are energized by opposite-phase currents to brake the carriage. It is, however, difficult to stop the carriage accurately at a desired position only by the braking operation due to the opposite-phase currents. For this reason, the conventional automatic transportation system of this kind is provided with a positioning apparatus which produces, after the speed of the carriage is sufficiently decreased by the opposite-phase currents, a magnetic attractive force to stop the carriage precisely at predetermined positions.
FIGS. 1 and 2 show the structure of the conventional automatic transportation system with such positioning apparatus. The system comprises two rows of roller supports 1 mounted on the floor, each row of roller supports being spaced by a predetermined distance from one another along a path or a track T. The roller supports 1 are provided at their tops with rollers 2, respectively, which are disposed at the same level above the floor. Placed on the rollers 2 for slidable movement is a carriage 3 on which goods to be transported are placed. Also provided along the track T at the sides thereof are guide rollers 4 which rollingly engage with side faces of the carriage 3 and prevent the carriage 3 from moving transversely of the track T. The carriage 3 has at a bottom surface thereof a reaction plate 5a of a LIM in the form of an elongated plate made of conductive materials. A plurality of stators 5b are mounted on the floor along the track T at equal intervals in such a manner that a top surface of each stator 5b faces a lower surface of the reaction plate 5a when the carriage 3 passes past the stator 5b. A pair of yokes 6a each having an E-shaped cross-section are fixedly mounted on the carriage 3 at its lower surface in such a manner that leg portions thereof extend downward, the yokes 6a being spaced from each other transversely of the carriage 3. A pair of yokes 6b each having an E-shaped cross-section are mounted on the floor at each predetermined stop position of the carriage 3 with their leg portions extending upward. In this case, end faces of the leg portions of the yokes 6b face those of the yokes 6a when the carriage 3 is disposed at the stop position. An induction coil 6c is wound around the center leg of each of the yokes 6b. In this automatic transportation system, the yokes 6a, the yokes 6b and the coils 6c constitute a positioning apparatus 6 for this automatic transportation system.
With this conventional system, the carriage 3 running at a high speed in the forward direction indicated by an arrow A in FIG. 1 along the track T is braked by a traveling magnetic field produced by the stator 5b in the reverse direction indicated by an arrow B, and approaches the stop position shown in FIG. 1 at a decreased speed. Then, when the arrival of the carriage 3 at the stop position is detected by means of a limit switch (not shown), the stator 5b is de-energized and at the same time the coil 6c of the positioning apparatus 6 is energized. As a result, a magnetic attractive force is exerted between the yokes 6a and the yokes 6b, so that the carriage 3 is positioned at the stop position with the top faces of the leg portions of the yokes 6b facing the bottom faces of the leg portions of the yokes 6a. In this case, a centripetal force P to decrease the deviation of the yokes 6a from the yokes 6b is exerted in the yokes 6 a. The magnitude of this centripetal force P varies, as shown in FIG. 3, in accordance with the amount of deviation X of the yokes 6a from the yokes 6b. As will be appreciated from FIG. 3, on condition that the deviation X is within a specific range L shown in FIG. 3, the greater the absolute value of the deviation X becomes, the greater the centripetal force P to decease the deviation X becomes, and the centripetal force P is rendered zero when the deviation X is zero.
With the above system, however, if there exists a friction force between the carriage 3 and the track T, the carriage 3 suffers from a non-negligible positioning error. For example, if a friction force M (FIG. 3) to suppress the rotation of the rollers 2 is exerted in the bearings of the rollers 2, the carriage 3 is subject to a positioning error of within the range of .+-.l/2 determined by the magnitude of the friction force M, since the centripetal force P becomes smaller than the friction force M within the range of .+-.l/2. Thus, the greater the friction force M is, the greater the positioning error of the carriage 3 becomes. The above-described conventional system is thus deficient in that the positioning error of the carriage 3 is not negligible and varies in accordance with the variation of the friction force M due to the aged-deterioration of the bearings and other components parts.